1. Field of the Invention
This invention relates to a technique of forming an ohmic contact to a two-dimensional carrier which is formed on a heterojunction interface, and more particularly to a semiconductor device which can reduce a source-gate resistance or a parasitic base resistance and can operate at a high operation speed, and fabrication method of such a semiconductor device.
2. Description of the Prior Art
The technical concept of using two-dimensional electron gas (hereinafter called "2DEG") formed on a heterojunction interface between n-type AlGaAs (aluminum gallium arsenide) and undoped GaAs (gallium arsenide) for the active layer of a field effect transistor (FET) is disclosed, for example, in Japanese Patent Laid-Open No. 160473/1980. Such an FET is generally called "two-dimensional electron gas field effect transistor (2DEG-FET)". The greatest problem for accomplishing high performance of 2DEG-FET is the reduction of a source-gate resistance (R.sub.sg). FIG. 2 (a) of the accompanying drawings is a sectional view of 2DEG-FET having a conventional structure and FIG. 2(b) is a band diagram of a conduction band energy in the proximity of a source electrode.
Namely, 1 .mu.m-thick undoped GaAs 11, about 50 nm-thick n-type Al.sub.x Ga.sub.1-x As(x.about.0.3) 12 and 50 nm-thick n.sup.+ GaAs 13 are formed on a semi-insulating GaAs substrate 10. A gate electrode 22 forms a Schottky junction with n-type AlGaAs 12 while source-drain electrodes 20, 21 form an alloy junction with n.sup.+ GaAs 13. Here, symbol .phi..sub.Bn represents the Schottky barrier height between the source electrode 20 and n.sup.+ GaAs 13.
FIG. 2(b) shows the conduction energy band diagram in the proximity of the source electrode 20 (A - A'). Unlike the ohmic contact of ordinary GaAs MESFET (Metal Semiconductor Field Effect Transistor), the heterojunction is formed between n.sup.+ GaAs 13 and n-type AlGaAs 12 in the case of 2DEG-FET and the heterojunction is formed also between n-type AlGaAs 12 and undoped GaAs 11. Therefore, a potential barrier .DELTA.E.sub.c is formed between 2DEG 14 and the ohmic metal 20 and specific contact resistance R.sub.c occurs between n.sup.+ GaAs 13 and 2DEG, thereby causing a critical problem for reducing R.sub.sg. The specific contact resistance R.sub.c.sup.TLM between the source electrode 20 and n.sup.+ GaAs 13 alone is the problem in the case of ordinary GaAs MESFETs, but the reduction of the specific contact resistance R.sub.c between the heterojunctions becomes the problem in the case of 2DEG-FET.
It is known of late that if heat-treatment is made by injecting Si ions, the heterojunction interface becomes broken down and AlGaAs/GaAs system superlattice becomes disordered, as discussed in the papers of "Opto-Electronics Joint Research Laboratory, 13th Conference" (1986), pp. 25-33. This disorder is believed to result from mutual diffusion of Ga and Al due to diffusion of Si.
As one of the methods of reducing R.sub.sg, a method which injects Si ions exhibiting the n type into the source-drain region and activates it has been employed. However, this method involves the problem that a high temperature process around 800.degree. C. is necessary as the activation temperature and a layer below the gate electrode becomes disordered, also. If a refractory gate metal is used as a masking material for the ion injection, the Si ions are diffused below the two-dimensional electron gas so that the shift of the threshold voltage (V.sub.th) of the transistor to the negative side, or so-called "short channel effect", becomes the problem.
In addition, the impurity distribution in AlGaAs 12 changes through the high temperature heating process so that V.sub.th shifts, as well.
On the other hand, heterojunction bipolar transistors (hereinafter called "HBTs") of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) have been fabricated conventionally in accordance with crystal growth methods such as MBE (Molecular Beam Epitaxy) or MODVD (Metal Organic Vapor Phase Epitaxy) comprising the steps of sequentially growing crystallographically n-type GaAs (collector), p.sup.+ GaAs (base), n-type AlGaAs (emitter) and n.sup.+ GaAs (cap layer) and then forming the emitter region, the base region, the base electrode, the collector electrode, the collector region, and so forth. (Refer, for example, to "IEDM Technical Digest 1985", pp. 328-331.) As is well known in the art, however, the emitter concentration, i.e. 10.sup.20 cm.sup.-3, is by far higher than the base concentration, i.e. 10.sup.18 cm.sup.-3, in the case of Si bipolar transistors. Accordingly, the emitter region can be formed by ion implantation after the formation of the base region and extremely fine n-p junction region can be realized. As a result, an extremely fine emitter can be formed. (Refer, for example, to "Electronics Lett.", Vol. 19, No. 8, april, 1983, p. 283.)
In contrast, in the case of HBT, the impurity concentration of the base region is ordinarily from 10.sup.18 to 10.sup.19 cm.sup.-3 while the AlGaAs impurity concentration of the emitter region is ordinarily from 10.sup.17 to 10.sup.18 cm.sup.-3 in order to reduce the base resistance and because the upper limit exists to the emitter concentration. In other words, the impurity concentration of the emitter region is ordinarily lower than that of the base region.
Therefore, unlike the Si bipolar transistors, HBT involves the problem that the emitter region cannot be formed by ion implantation after the base region is formed.
Since the emitter region is formed by epitaxial growth process such as MBE/MOCVD or the like, it has been extremely difficult to etch finely (.about.0.5 .mu.m level) the emitter region unlike ion implantation.